Click Entire disk and choose the disk or disks you'd like to erase, and then select an erasing algorithm for the disk(s) and Click Erase Now. (For system disk erasing, you need to boot from CD/DVD/USB).
Erase disk, Erase hard drive.
Prepare a bland CD/DVD/USB and click Bootable media, follow the guide to create the ISO file and then burn it the blank media, restart the computer and boot from CD/USB to erase system disk or system drive.
Boot CD erase tool, eraser bootable, eraser bootable USB